The present invention relates to a circuit for controlling the "on" and "off" operation of a PNP transistor. More particularly, the present invention relates to a circuit for reducing the delay in the transition time of a PNP transistor caused by stored charge in the transistor's base as the transistor changes from a conducting state to a non-conducting state.
PNP transistors are often employed as switching elements in integrated switching regulator circuits. Typically, a PNP transistor used in this fashion has its emitter connected to the positive supply voltage. The transistor is controllably cycled "on" and "off" to regulate the flow of current between its emitter and collector by intermittently supplying a forward biasing or drive current to the transistor. The transistor is turned "on" by a base drive current, which typically drives the transistor into saturation. The transistor is turned "off" by removing the base drive current.
A problem, however, is that an amount of charge is stored in the PNP transistor's base when the transistor is turned "on". This base charge causes a delay in turning the transistor "off" because the stored base charge causes the base-emitter junction of the transistor to remain forward biased for a period of time following the removal of the base drive current. This delay in turning the transistor "off" may be several microseconds long, particularly if the transistor had been driven into saturation as typically occurs in a switching regulator. This delay limits the switching speed of the PNP transistor and, as a result, the efficiency of the circuit.
The time required to turn a switching PNP transistor "off" may be reduced by applying a reverse drive current to the base of the transistor to discharge the stored base charge. The greater the reverse drive current, the faster the base charge will be discharged and the faster the transistor will turn "off." To drive the PNP transistor quickly into a non-conducting state, the reverse drive current must be able to drive the base voltage of the PNP transistor close to the level of the voltage at its emitter.
A circuit for decreasing the switching time of a PNP transistor is shown in O'Neill and Nelson U.S. Pat. No. 4,487,520. The present invention provides alternative circuits for generating a reverse drive current to speed up the switching speed of a PNP transistor from the "on" state to the "off" state.